Ferrites are a class of compounds composed of oxidized iron and other metals in a brittle ceramic state. They are polycrystalline, meaning that they are composed of large amounts of minute crystals, and they exhibit strong magnetic properties. A common use for ferrites is in the suppression of electromagnetic interference (EMI) and radio frequency (RF) interference in electrical circuits, where they are often referred to as magnetic insulators.

Two general categories for the composition of ferrites exists. Soft ferrite compounds are a mixture of iron and lightweight metals such as nickel, aluminum, or manganese, where they are used in electrical transformers and other devices that require the ability for the magnetic field to be easily reversed. Hard ferrite compounds are composed of iron and harder metals, such as cobalt, barium, and strontium. Barium ferrite compounds have uses as magnetic insulators and where permanent magnets are required in consumer applications, such as magnetic door latches.

The use of ferrite material is widespread, as they are easy and inexpensive to manufacture. Their main attraction is that they demonstrate large magnetic flux densities compared to the small magnetizing forces applied to them. Their frequent use leads to various trade names for diverse applications, with EMI suppression ferrites often being called magic beads, due to the lump-like appearance that they can have when attached to electrical wiring.

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In RF and ambient electrical signal suppression, ferrites are most effective at bandwidth levels above 100 megahertz, where they replace decoupling capacitors that begin to exhibit circuit resonance problems in noise filtering above 75 megahertz. They can be designed to impede low frequencies below 10 megahertz as well. This makes ferrites useful as both alternating current (AC) and direct current (DC) noise filters.

Ferrite cores are designed to be as thick and long as is practical for the wiring or device into which they are built. This requires that, when used on electrical cable, they be encased in plastic or heat-shrink tubing that prevents them from breaking up under stress due to their fragile ceramic nature. The ferrites used for EMI suppression also tend to be of the hard type, which makes them more prone to breakage than their soft counterparts. A ferrite core often is employed to shield 100 Base-T cabling used in computer networking, which can be subject to a significant amount of shock during installation and maintenance. Small amounts of damage to a ferrite shield, however, will not degrade its ability to filter out noise.

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